The invention pertains to an impermeable elastic membrane of the type which is used in hydropneumatic accumulators to separate gas and liquid compartments.
European Patent EP0360648 describes such a membrane, and, in particular, a sandwich membrane, which is comprised of a film of gas impermeable material, preferably polyvinyl alcohol, included between two layers of an elastic material such as polyurethane.
The polyvinyl alcohol films which are used, especially in the field of packing materials, are obtained by lamination in a thickness from 25 to 100 microns. These films are not sufficiently resistant to be used for making hydropneumatic accumulator membranes which operate at pressures on the order of 200 bars.
In order to obtain films of sufficient thickness, on the order of 450 to 1400 microns, successive layers are produced by laminating a large number of thin films as mentioned in European Patent EP0482916.
On the other hand, it is known that the thick partially hydrolyzed polyvinyl alcohol films which are generally used, obtained by lamination or by extrusion, are porous because of the escape of volatile substances caused by heating in a non-anhydrous medium.
The escaping gases are produced according to the following mechanism: ##STR1##